As patterns that are formed in semiconductor devices are downscaled, the wavelength of the exposure light is becoming shorter; and the practical use of EUV exposure which utilizes extreme ultraviolet (EUV) light of a wavelength of 13.5 nm is needed even more.
Because EUV light has a short wavelength and a reflective optical system is used as the projection optical system toward the wafer, stray light (flare) easily occurs in EUV exposure due to the scattered light caused by the unevenness of the mirror surfaces of the reflective optical system.